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      <h2><a href="./">Go by Example 中文版</a>: 通道选择器</h2>
      
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            <p>Go 的 <em>选择器（select）</em>  让你可以同时等待多个通道操作。
将协程、通道和选择器结合，是 Go 的一个强大特性。</p>

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            <a href="http://play.golang.org/p/AC0FmDQsy_o"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
          <div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>

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          <div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
    <span class="s">&quot;fmt&quot;</span>
    <span class="s">&quot;time&quot;</span>
<span class="p">)</span>
</pre></div>

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          <div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
</pre></div>

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            <p>在这个例子中，我们将从两个通道中选择。</p>

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          <div class="highlight"><pre>    <span class="nx">c1</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">string</span><span class="p">)</span>
    <span class="nx">c2</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">string</span><span class="p">)</span>
</pre></div>

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            <p>各个通道将在一定时间后接收一个值，
通过这种方式来模拟并行的协程执行（例如，RPC 操作）时造成的阻塞（耗时）。</p>

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          <div class="highlight"><pre>    <span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
        <span class="nx">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="mi">1</span> <span class="o">*</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Second</span><span class="p">)</span>
        <span class="nx">c1</span> <span class="o">&lt;-</span> <span class="s">&quot;one&quot;</span>
    <span class="p">}()</span>
    <span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
        <span class="nx">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="mi">2</span> <span class="o">*</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Second</span><span class="p">)</span>
        <span class="nx">c2</span> <span class="o">&lt;-</span> <span class="s">&quot;two&quot;</span>
    <span class="p">}()</span>
</pre></div>

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            <p>我们使用 <code>select</code> 关键字来同时等待这两个值，
并打印各自接收到的值。</p>

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          <div class="highlight"><pre>    <span class="k">for</span> <span class="nx">i</span> <span class="o">:=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">i</span> <span class="p">&lt;</span> <span class="mi">2</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span> <span class="p">{</span>
        <span class="k">select</span> <span class="p">{</span>
        <span class="k">case</span> <span class="nx">msg1</span> <span class="o">:=</span> <span class="o">&lt;-</span><span class="nx">c1</span><span class="p">:</span>
            <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;received&quot;</span><span class="p">,</span> <span class="nx">msg1</span><span class="p">)</span>
        <span class="k">case</span> <span class="nx">msg2</span> <span class="o">:=</span> <span class="o">&lt;-</span><span class="nx">c2</span><span class="p">:</span>
            <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;received&quot;</span><span class="p">,</span> <span class="nx">msg2</span><span class="p">)</span>
        <span class="p">}</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>

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            <p>跟预期的一样，我们首先接收到值 <code>&quot;one&quot;</code>，然后是 <code>&quot;two&quot;</code>。</p>

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          <div class="highlight"><pre><span class="gp">$</span> <span class="nb">time</span> go run <span class="k">select</span>.go 
<span class="go">received one</span>
<span class="go">received two</span>
</pre></div>

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            <p>注意，程序总共仅运行了两秒左右。因为 1 秒 和 2 秒的 <code>Sleeps</code> 是并发执行的，</p>

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          <div class="highlight"><pre><span class="go">real    0m2.245s</span>
</pre></div>

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